Technologies that combine and unify the MEMS and small sized optical elements have vigorously been developed recently. Various tunable filters making the wavelength of resonating light changed have been proposed upon controlling and changing the distance of mirrors in the resonator constituted of a pair of optical mirrors located facing to each other in parallel by means of the MEMS, in the optical filter having the Fabry-Perot interferometer (FPI) structure. For example, aiming at application to the optical communication by the wavelength division multiplex (WDM) system, tunable devices for infrared rays in the wavelength band of C band (1.55 μm band), L band (1.58 μm band) and S band (1.49 μm band) have been exhibited (For example, refer to JP2003-101138).
The tunable filter in this case has a structure in which a silicon semiconductor or a compound semiconductor having the light transmittance state to the infrared ray band is employed as a material for the mirror structure body, and a pair of mirror structure bodies and the distance changing mechanism of mirror surfaces are integrally formed on the semiconductor substrate. Here, the pair of the mirror structure bodies is formed in such a manner that the mirror surfaces thereof are perpendicular to the surface of the substrate mentioned above, and one of the mirror structure bodies can move in the direction parallel to the substrate surface by means of the distance changing mechanism formed on the same substrate surface. However, such a tunable filter has a problem that the substrate mentioned above is not transparent to visible light and ultraviolet rays, so that it cannot be applied to a wide wavelength range of visible radiation including these wavelength bands.
Given this factor, a tunable filter of the FPI structure applicable to the light of wide range band including visible radiation has been proposed (For example, refer to JP2005-338534). For the tunable filter in this case, a first substrate of silicon and a second substrate having the light transmissive state to the wide wavelength range are laminated together. For a resonator comprising a first reflection film provided on the opening portion penetrating a specific position of the first substrate and a second reflection film located at the position of the second substrate facing to the first reflection film, impingement of the input light and outgoing of the output light are to be carried out with reference to the direction perpendicular to the first substrate surface and the second substrate surface as the optical axis.
Here, in order to change the distance between the first reflection film and the second reflection film, the first reflection film is formed on the movable portion of the first substrate, and the movable portion is to shift in the direction perpendicular to the surface of the substrate by means of the actuator constituted of so-called the parallel flat type electrostatic actuator. In this case, though application to the light of a wide wavelength band is possible, driving the actuator making the first reflection film move vertically becomes hard at a low voltage.
In addition, a tunable filter of the FPI structure has been indicated wherein each end surface of a pair of optical fibers, which are light guides, is positioned facing to each other as the mirror surfaces of the resonator mentioned above, and the distance between the end surfaces of the optical fibers is controlled by a parallel plane type electrostatic actuator (For example, refer to JP2006-525510). The tunable filter in this case can also be applied to a light of wide wavelength range. However in this case, miniaturization of the MEMS as well as low voltage driving of the actuator will become difficult.